Noise mitigation remains a big and common issue in our society, although many progresses have been achieved in past decades. Several types of industrial sound absorbing materials or structures are available for noise cancelling, ranging from open-cell foams, fibrous materials, and perforated panels. However, the challenges of cancelling noise still exist in a variety of environments, especially when dimension and self-weight of the sound absorbers are sensitive concerns. For example, the noise inside an airplane is a main source that affects flight comfort, but to maximize fuel economy, it is impracticable to remove the noise by using above mentioned regular sound absorbers, because they require excessively large space to generate significant dissipation for low frequency noises.
The prospect of delivering a satisfied sound absorption by conventional sound absorbers with a limited thickness is still not optimistic. It is widely recognized that the thickness of ¼ wavelength is a precondition to achieve full sound absorption by conventional sound absorbers. Generally, sound absorbers are porous materials and structures, and usually installed on or attached to a rigid surface with or without a designated distance. When the airborne sound waves impinge onto the absorbers, pressurized airflow is driven to penetrate into the pores of the absorbers and moves through the walls within the absorbers. Therefore, viscous frictions are generated and acoustic energy is converted into heat. However, this process happens efficiently only when the path of airflow inside the absorbers is sufficiently long, i.e. the absorbers are sufficiently thick. To cancel a common sound of 500 Hz, the required absorbers may be as thick as 170 mm, which greatly restricts their application.
Although there may exist one possible approach to overcome the limitation of ¼ wavelength, which is adopting resonance, such as Helmholtz resonance, the narrow bandwidth nature of the Helmholtz resonance greatly sets back its popularization. Various sound absorbers containing Helmholtz resonators have been designed for purpose of noise absorption, e.g., Helmholtz absorber containing extended necks, Helmholtz absorber comprising tuneable sized cavities, or even combination of Helmholtz absorber and porous materials. Due to the strict condition for generating Helmholtz resonance, almost all Helmholtz resonator based absorbers are effective in a narrow frequency band. People may combine a group of Helmholtz resonators with different sizes to broaden the frequency band, however, most designs of this kind would have significantly reduced the amplitude of sound absorption, because a reduction of porosity may happen to each individual Helmholtz resonator when a combination of several Helmholtz resonators is used.
In addition, conventional sound absorbers generally do not present aesthetically pleasing exteriors due to their porous nature. The sound absorbers may be installed behind acoustic transparent facings capable of preserving acceptable colourful images. However, the acoustic facings, typically fabrics, still cause attenuations to lights and thus are less likely to be widely favoured for decoration. Other facings such as perforated panels also have limitations as they are not fully acoustic transparent and may pose restrictions on the bandwidth of the sound absorbers.